Capsaicin, a pungent substance derived from the plants of the solanaceae family (hot chili peppers) has long been used as an experimental tool because of its selective action on the small diameter afferent nerve fibers (C fibers and A-delta fibers) that are believed to signal pain. From studies in animals, capsaicin appears to trigger C fiber membrane depolarization by opening cation channels permeable to calcium and sodium. Recently one of the receptors for capsaicin effects has been cloned.
Although detailed mechanisms are not yet known, capsaicin mediated effects include: (i) activation of nociceptors in peripheral tissues; (ii) eventual desensitization of peripheral nociceptors to one or more stimulus modalities; (iii) cellular degeneration of sensitive A-delta and C fiber afferents; (iv) activation of neuronal proteases; (v) blockage of axonal transport; and (vi) the decrease of the absolute number of nociceptive fibers without affecting the number of non-nociceptive fibers.
Because of capsaicin's ability to desensitize nociceptors in peripheral tissues, its potential analgesic effects have been assessed in various clinical trials. However, since the application of capsaicin itself frequently causes burning pain and hyperalgesia apart from the neuropathic pain being treated, patient compliance has been poor and the drop out rates during clinical trials have exceeded fifty percent. The spontaneous burning pain and hyperalgesia are believed to be due to intense activation and temporary sensitization of the peripheral nociceptors at the site of capsaicin application. This activation and sensitization occur prior to the desensitization phase. The activation phase could be a barrier to use of capsaicin because of the pain produced.
It would be advantageous to provide methods and compositions including capsaicin or analogues thereof with effective concentrations to cause an analgesic effect without the side effects normally associated with the use of capsaicin.
It is therefore an object of the present invention to provide a method for using capsaicin or capsaicin analogues at high concentrations with a prolonged effect.